# A torque of $20\; Nm$ acting on a rigid body produces are angular acceleration of $4 rad /s^2$ What is the moment of inertia of the rigid body rotating about the fixed axis?

$\begin {array} {1 1} (a)\;5\;kg\;m^2 & \quad (b)\;4\;kg\;m^2 \\ (c)\;80\;kg\;m^2 & \quad (d)\;12\;kg\;m^2 \end {array}$

$z= I \alpha$
$I= \large\frac{z}{\alpha}$
$\quad=\large\frac{20}{4}$
$\quad= 5 \;kg\;m^2$
hence a is the correct answer
edited Jun 18, 2014